How to change the Camera mesh?
You can overlay the 3D Viewport using a draw handler.

Use 'PRE_VIEW' to have it in world space behind everything else, 'POST_VIEW' for world space like a real object (depth test). 'POST_PIXEL' is screen-space on top of everything else, but requires 2D coordinates.

You would need to hide the real camera however, which is drawn by C code - maybe put it on another layer or simply hide it? Note that you can't move the camera while it's hidden.

Proof of concept:

import bpy
import bgl
import blf
from mathutils import Vector

bbox_normal = ((-1.0, -1.0, -1.0), (-1.0, -1.0, 1.0), (-1.0, 1.0, 1.0), (-1.0, 1.0, -1.0), (1.0, -1.0, -1.0), (1.0, -1.0, 1.0), (1.0, 1.0, 1.0), (1.0, 1.0, -1.0))

def draw_callback_px(self, context):
    if context.scene.camera is None:
        return

    font_id = 0  # XXX, need to find out how best to get this.

    # 50% alpha, 2 pixel width line
    bgl.glEnable(bgl.GL_BLEND)
    bgl.glColor4f(1.0, 0.0, 0.0, 0.5)
    bgl.glLineWidth(2)

    bbox = [context.scene.camera.matrix_world * Vector(x) for x in bbox_normal]

    bgl.glBegin(bgl.GL_LINE_STRIP)
    bgl.glVertex3f(*bbox[0])
    bgl.glVertex3f(*bbox[1])
    bgl.glVertex3f(*bbox[2])
    bgl.glVertex3f(*bbox[3])
    bgl.glVertex3f(*bbox[0])
    bgl.glVertex3f(*bbox[4])
    bgl.glVertex3f(*bbox[5])
    bgl.glVertex3f(*bbox[6])
    bgl.glVertex3f(*bbox[7])
    bgl.glVertex3f(*bbox[4])
    bgl.glEnd()

    bgl.glBegin(bgl.GL_LINES)
    bgl.glVertex3f(*bbox[1])
    bgl.glVertex3f(*bbox[5])
    bgl.glVertex3f(*bbox[2])
    bgl.glVertex3f(*bbox[6])
    bgl.glVertex3f(*bbox[3])
    bgl.glVertex3f(*bbox[7])
    bgl.glEnd()


    # restore opengl defaults
    bgl.glLineWidth(1)
    bgl.glDisable(bgl.GL_BLEND)
    bgl.glColor4f(0.0, 0.0, 0.0, 1.0)


class ModalDrawOperator(bpy.types.Operator):
    """Draw a line with the mouse"""
    bl_idname = "view3d.modal_operator"
    bl_label = "Simple Modal View3D Operator"

    def modal(self, context, event):
        context.area.tag_redraw()

        if event.type == 'ESC':
            bpy.types.SpaceView3D.draw_handler_remove(self._handle, 'WINDOW')
            return {'CANCELLED'}

        return {'PASS_THROUGH'}

    def invoke(self, context, event):
        if context.area.type == 'VIEW_3D':
            # the arguments we pass the the callback
            args = (self, context)
            # Add the region OpenGL drawing callback
            # draw in view space with 'POST_VIEW' and 'PRE_VIEW'
            self._handle = bpy.types.SpaceView3D.draw_handler_add(draw_callback_px, args, 'WINDOW', 'PRE_VIEW')
            if context.scene.camera:
                context.scene.camera.hide = True
            context.window_manager.modal_handler_add(self)
            return {'RUNNING_MODAL'}
        else:
            self.report({'WARNING'}, "View3D not found, cannot run operator")
            return {'CANCELLED'}


def register():
    bpy.utils.register_class(ModalDrawOperator)


def unregister():
    bpy.utils.unregister_class(ModalDrawOperator)

if __name__ == "__main__":
    register()

    for area in bpy.context.screen.areas:
        if area.type == 'VIEW_3D':
            ctx = bpy.context.copy()
            ctx['area'] = area
            bpy.ops.view3d.modal_operator(ctx, 'INVOKE_DEFAULT')
            break


To draw the box on top of everything else (like X-ray):

from bpy_extras.view3d_utils import location_3d_to_region_2d

# ...

    bbox = []
    region = context.region
    rv3d = context.space_data.region_3d
    for x in bbox_normal:
        coord = context.scene.camera.matrix_world * Vector(x)
        bbox.append(tuple(map(round, location_3d_to_region_2d(region, rv3d, coord))))

    bgl.glBegin(bgl.GL_LINE_STRIP)
    bgl.glVertex2i(*bbox[0]) # must be 2i for 2D integer values!

    # ...